Heat pump with improved defrost cycle and method of defrosting a heat exchanger

ABSTRACT

Heat pumps with improved defrost cycles, methods of defrosting heat exchangers, and methods of improving the effectiveness of defrost cycles of heat pumps, for example, having a microchannel outdoor heat exchanger. A defrost valve in a refrigerant conduit opens during a defrost cycle to deliver hot refrigerant gas to a portion of the heat exchanger that otherwise defrosts more slowly or less completely than other portions of the heat exchanger. Particular embodiments pass hot refrigerant gas through a header of the heat exchanger, such as a bottom header. In a number of embodiments, the defrost valve is open only during a portion of the defrost cycle. Further, in some embodiments, the fan that is used to blow air through the heat exchanger is operated in a reversed direction during at least part of the defrost cycle to counteract natural convection through the heat exchanger.

FIELD OF THE INVENTION

This invention relates to defrost cycles for heat pumps and methods ofdefrosting heat exchangers. Particular embodiments concern heat pumps,defrost cycles, and defrost methods for heat pumps with microchanneloutdoor coils.

BACKGROUND OF THE INVENTION

Heat pump HVAC units have been used for some time to heat and coolspaces that people occupy such as the interior of buildings. Heat pumpshave also been used for other purposes such as heating water andproviding heat for industrial processes. Heat pumps are typically moreefficient than alternative heat sources, such as electrical resistanceheating, because heat pumps extract heat from another source, such asthe environment, in addition to providing heat produced from theconsumption of electrical power. As a result, heat pumps often reduceenergy consumption in comparison with alternatives.

More broadly speaking, a heat pump is a machine or device that transfersthermal energy from one location, at a lower temperature, to anotherlocation, which is at a higher temperature. Accordingly, heat pumps movethermal energy in a direction opposite to the direction that it normallyflows. Thus, air conditioners and freezers are also types of heat pumps,as used herein. Some, types of heat pumps are dedicated to refrigerationonly, some types are dedicated to heat only, and some types perform bothfunctions, for instance, depending on whether heating or cooling isneeded at the time. Further, in some applications, the heating and thecooling are both put to beneficial use at the same time.

In many applications, heat pumps extract heat from air, such as outdoorair, when a heat pump is being used to provide heat. In other examples,heat pumps extract heat from air that is being cooled such as air in afreezer when the heat pump is being used to cool the freezer. When aheat pump is used to extract heat from outdoor air, if outdoor airtemperatures are close to or below freezing, moisture in the air canfreeze or desublimate onto the outdoor air heat exchanger of the heatpump, forming frost on the heat exchanger. The same may occur on a heatexchanger used to cool a freezer or refrigerator, as other examples.Build up of frost on the heat exchanger can interfere with heat transferfrom the air to the refrigerant in the heat pump. Specifically, frostcan insulate the heat exchanger, or can even block air flow through theheat exchanger. To address this problem, heat pumps have been operatedin a defrost mode during a brief defrost cycle, during which the heatexchanger is warmed to melt the frost.

For example, heat pumps that are used in an HVAC application to heat andcool a building, when being used in a heating mode, may interrupt theheating mode periodically to run a defrost cycle. In the defrost cycle,the heat pump may be operated in the cooling mode, except without theoutdoor air fan running. In this mode, hot refrigerant gas is deliveredto the outdoor air heat exchanger heating the heat exchanger and meltingfrost that has accumulated on the heat exchanger. After the defrostcycle has been completed, the heat pump returns to the heating modeuntil another defrost cycle is initiated.

In recent years, microchannel heat exchangers have replaced other typesof heat exchangers in various applications including automobile airconditioning. Microchannel heat exchangers typically have a firstheader, a second header, and multiple cross tubes extending from thefirst header to the second header. See U.S. patent application Ser. No.12/561,178, Publication 2010/0071868, for example. Usually, each of themultiple cross tubes connects to the first header and each of themultiple cross tubes connects to the second header. Moreover, inmicrochannel heat exchangers, the first header is often parallel to thesecond header, the multiple cross tubes are often parallel to eachother, the headers are often perpendicular to the cross tubes, and themultiple cross tubes typically each include multiple contiguous parallelrefrigerant passageways therethrough (e.g., extending from the firstheader to the second header). These refrigerant passageways aretypically smaller than refrigerant passageways in prior heat exchangerdesigns, which is the origin of the name “microchannel”. Furthermore,most microchannel heat exchangers include multiple fins between thecross tubes, and the fins are typically bonded to the cross tubes.Microchannel heat exchangers generally offer a relatively higheffectiveness relative to their cost and the restriction that theyprovide, in comparison with prior heat exchangers used for similarpurposes.

Microchannel heat exchangers have also been used in place of other typesof heat exchangers in residential air conditioning units. In heat pumpHVAC units, however, it has been found that microchannel heat exchangersdo not defrost as well as certain prior heat exchangers. For example, ifduring a defrost cycle, hot refrigerant gas is introduced into the firstheader and travels though the cross tubes to the second header, thesecond header and the ends of the cross tubes that are connected to thesecond header often have not gotten warm enough to melt the frost therewithin a desired amount of time. As a result, frost or ice may remain onthis portion of the heat exchanger after the defrost cycle is ended, orit may be necessary to extend the defrost cycle and remain in thedefrost mode for a longer time.

In various applications, in the defrost mode, as hot refrigerant gas isdelivered to the heat exchanger, a portion of this heat will betransferred to the environment surrounding the heat exchanger. Inparticular, heat may be transferred via convection to air around theheat exchanger. Heat that is transferred to the air is less available todefrost the heat exchanger, especially for portions of the heatexchanger that are physically below the location where the heat istransferred to the air. As mentioned, in prior heat pumps, the outdoorair fan was typically turned off during the defrost cycle, which avoidsheat loss to the surrounding air through forced convection. Naturalconvection, has been found to occur, however, under such circumstances,carrying the hot air and heat upward where the heat is lost to theenvironment. For example, air heated by the heat exchanger can travelupward through the fan, pushed up by buoyancy forces from denser colderair, and colder air tends to flow through the heat exchanger to replacethe warm air that has risen. This colder air flowing through the heatexchanger continues to cool the heat exchanger, cooling the refrigerantand taking heat away from the intended purpose of melting the frost. Asa result, frost may remain on the heat exchanger, particularly on thelower portion of the heat exchanger, after the defrost cycle iscompleted, or it may be necessary to extend the defrost cycle and remainin the defrost mode for a longer time in order to defrost the heatexchanger completely or adequately.

Extending the defrost cycle in HVAC applications, for example, isundesirable because the heat pump delivers cold air to the space duringthe defrost cycle, which may lower the temperature in the spacesignificantly below the thermostat set point temperature, may cause acold draft and discomfort to the occupants of the space during thedefrost mode, may cause the occupants of the space to believe that theheat pump is not working properly, or a combination thereof, forinstance. Extension of defrost cycles and less effective defrost cyclesmay be undesirable in other applications (besides HVAC) as well, amongother things, because heat or cooling is unavailable during the defrostcycle and because energy used during the defrost cycle does notcontribute to the heat or cooling that is intended to be produced by theheat pump.

As a result, needs or potential for benefit or improvement exist fordefrost cycles for heat pumps, and methods of defrosting heat exchangersof heat pumps, that are more effective, that direct hot refrigerant gasto areas of the heat exchanger that otherwise would not get warm enough,that take less time to complete, that work effectively with microchannelheat exchangers, or a combination thereof, as examples. In addition,needs or potential for benefit or improvement exist for defrost cyclesfor heat pumps, and methods of defrosting heat exchangers, that reducethe amount of heat loss to the air from the heat exchanger during thedefrost cycle, that reduce natural convection during the defrost cycle,or a combination thereof, as examples. Needs and potential for benefitor improvement also exist for heat pumps and methods of defrosting heatexchangers that that are inexpensive, that can be readily manufactured,that are easy to install, that are reliable, that have a long life, thatare compact, that are efficient, that can withstand extremeenvironmental conditions, or a combination thereof, as examples.

Further, needs or potential for benefit or improvement exist for methodsof controlling, manufacturing, and distributing such heat pumps, HVACunits, buildings, systems, devices, and apparatuses. Other needs orpotential for benefit or improvement may also be described herein orknown in the HVAC or heat pump industries. Room for improvement existsover the prior art in these and other areas that may be apparent to aperson of ordinary skill in the art having studied this document.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a heat pump havingan improved defrost cycle;

FIG. 2 is a side view of an example of a heat exchanger, for instance,of the heat pump illustrated in FIG. 1;

FIG. 3 is an isometric view showing part of a heat exchanger, such asthe heat exchanger shown in FIG. 2;

FIG. 4 is a side view of another example of a heat exchanger, forinstance, of the heat pump illustrated in FIG. 1, this example having anextension tube;

FIG. 5 is an isometric view of an outdoor HVAC unit for a split HVACsystem; and

FIG. 6 is a flow chart illustrating an example of a method of defrostinga first heat exchanger of a heat pump and a method of improving theeffectiveness of a defrost cycle of a heat pump.

These drawings illustrate, among other things, examples of certainaspects of particular embodiments of the invention. Other embodimentsmay differ. Various embodiments may include aspects shown in thedrawings, described in the specification, shown or described in otherdocuments that are incorporated by reference, known in the art, or acombination thereof, as examples.

SUMMARY OF PARTICULAR EMBODIMENTS OF THE INVENTION

This invention provides, among other things, heat pumps with improveddefrost cycles, methods of defrosting heat exchangers, and methods ofimproving the effectiveness of defrost cycles of heat pumps. Variousembodiments include a defrost valve located in a refrigerant conduitthat opens during a defrost cycle to deliver hot refrigerant gas to aportion of the heat exchanger that otherwise defrosts more slowly orless completely than other portions of the heat exchanger. Particularembodiments pass hot refrigerant gas through a header of the heatexchanger. In a number of embodiments, the defrost valve is open onlyduring a portion of the defrost cycle. Further, in some embodiments, thefan that is used to blow air through the heat exchanger is operated in areversed direction during the defrost cycle to counteract naturalconvection through the heat exchanger.

Various embodiments provide, for example, as an object or benefit, thatthey partially or fully address or satisfy one or more of the needs,potential areas for benefit, or opportunities for improvement describedherein, or known in the art, as examples. Certain embodiments provide,for instance, heat pumps having improved defrost cycles, and methods ofdefrosting heat exchangers, that are more effective, that direct hotrefrigerant gas to areas of the heat exchanger that otherwise would notget warm enough, that take less time to complete, that worksatisfactorily with microchannel heat exchangers, or a combinationthereof, as examples. In addition, a number of embodiments providedefrost cycles for heat pumps, and methods of defrosting heatexchangers, that reduce the amount of heat loss to the air from the heatexchanger during the defrost cycle, that reduce natural convectionduring the defrost cycle, or a combination thereof, as examples. Variousembodiments are reasonably inexpensive, can be readily manufactured, areeasy to install, are reliable, have a long life, are compact, areefficient, can withstand extreme environmental conditions, or acombination thereof, as examples.

Specific embodiments of the invention provide various heat pumps havingimproved defrost cycles. In a number of embodiments, for example, eachheat pump includes a first heat exchanger, a compressor, at least oneexpansion device, a first refrigerant conduit, a second refrigerantconduit, a third refrigerant conduit, a defrost valve, and a controlsystem that controls the defrost valve. In various embodiments, forinstance, the first heat exchanger includes at least one firstconnection point, a second connection point, and a third connectionpoint. Further, in many of these embodiments, the first refrigerantconduit connects a discharge port on the compressor to the at least onefirst connection point of the first heat exchanger, the secondrefrigerant conduit connects the second connection point of the firstheat exchanger to the at least one expansion device, and the thirdrefrigerant conduit connects the first refrigerant conduit to the thirdconnection point of the first heat exchanger, for example. Moreover, ina number of embodiments, the defrost valve is located in the thirdrefrigerant conduit, for instance, between the first refrigerant conduitand the third connection point of the first heat exchanger. In variousembodiments, when the defrost valve is closed, refrigerant flow throughthe third refrigerant conduit is blocked. Even further, in a number ofembodiments, the control system opens the defrost valve during thedefrost cycle, for example, allowing refrigerant to flow through thethird refrigerant conduit to the third connection point to defrost thefirst heat exchanger between the third connection point and the secondconnection point, for instance, during at least part of the defrostcycle.

In some such embodiments, the first heat exchanger includes a firstheader, a second header, and multiple cross tubes extending from thefirst header to the second header. Further, in particular embodiments,each of the multiple cross tubes connects to the first header and eachof the multiple cross tubes connects to the second header. Moreover, incertain embodiments, the first header is parallel to the second header,the multiple cross tubes are parallel to each other, the multiple crosstubes each include multiple contiguous parallel refrigerant passagewaystherethrough, the first heat exchanger further includes multiple finsbetween the cross tubes, or a combination thereof, for example. The finscan be bonded to the cross tubes, for example. Further, in particularembodiments, the heat pump further includes an extension tube, forinstance, located within the second header. In certain embodiments, theextension tube within the second header is substantially parallel to thesecond header, the at least one first connection point to the first heatexchanger is at the first header, the second connection point to thefirst heat exchanger is at the second header, the third connection pointto the first heat exchanger is at the extension tube, or a combinationthereof, as examples.

In a number of embodiments, the at least one first connection point tothe first heat exchanger is at the first header, the second connectionpoint to the first heat exchanger is at the second header, the thirdconnection point to the first heat exchanger is at the second header, ora combination thereof, as examples. Moreover, in certain embodiments,the second header has a first end and a second end, each of the multiplecross tubes connects to the second header between the first end and thesecond end, the second connection point to the first heat exchanger isat the second end of the second header, the third connection point tothe first heat exchanger is at the first end of the second header, or acombination thereof, for instance. Furthermore, in some embodiments, thefirst header has a third end and a fourth end, each of the multiplecross tubes connects to the first header between the third end and thefourth end, the at least one first connection point to the first heatexchanger consists of a single first connection point at the third endof the first header, or a combination thereof, as examples. In otherembodiments, on the other hand, the at least one first connection pointto the first heat exchanger includes a primary first connection point tothe heat exchanger at the third end of the first header and a secondaryfirst connection point to the heat exchanger at the fourth end of thefirst header, and the first refrigerant conduit connects the dischargeport on the compressor to the primary first connection point and to thesecondary first connection point, as another example.

In various embodiments, the control system includes a digital controllerthat includes programming instructions to open the defrost valve, forinstance, during the defrost cycle. To defrost the first heat exchanger,for example, between the third connection point and the secondconnection point. Furthermore, in some such embodiments, the digitalcontroller further includes programming instructions to keep the defrostvalve closed when the heat pump is not in the defrost cycle. Inaddition, in some embodiments, the digital controller further includesprogramming instructions to keep the defrost valve closed during part ofthe defrost cycle, for example, to defrost the first heat exchangerbetween the at least one first connection point and the secondconnection point. Furthermore, the heat pump, in many embodiments,further includes a first fan positioned and configured to move airthrough the first heat exchanger. In certain embodiments, the controlsystem includes a digital controller that includes programminginstructions to operate the first fan in a reversed direction during atleast part of the defrost cycle to reduce natural convection through thefirst heat exchanger during the at least part of the defrost cycle.

In some embodiments, the heat pump further includes a reversing valvelocated, for example, in the first refrigerant conduit between thedischarge port on the compressor and the at least one first connectionpoint of the first heat exchanger. In particular embodiments, forinstance, the third refrigerant conduit connects to the firstrefrigerant conduit between the reversing valve and the at least onefirst connection point of the first heat exchanger. Moreover, in anumber of such embodiments, the heat pump further includes a second heatexchanger, a fourth refrigerant conduit connecting the at least oneexpansion device to the second heat exchanger, a fifth refrigerantconduit connecting the second heat exchanger to the reversing valve, anda sixth refrigerant conduit connecting the reversing valve to an inletport on the compressor, as examples.

Other specific embodiments of the invention provide various methods, forexample, of defrosting a first heat exchanger of a heat pump. Such aheat pump can include, for example, the first heat exchanger, acompressor, at least one expansion device, and a second heat exchanger,for instance. Moreover, in certain embodiments, the first heat exchangerincludes a first header, a second header, and multiple cross tubesextending from the first header to the second header, for example, eachof the cross tubes connecting to the first header and to the secondheader. In various embodiments, the first heat exchanger includes afirst connection point, a second connection point, and a thirdconnection point. Further, in particular embodiments, first connectionpoint is to the first header, the second connection point is to thesecond header, and the third connection point is also to the secondheader.

In a number of embodiments, such a method includes (e.g., in any orderexcept where a particular order is explicitly indicated), at leastcertain acts. Such acts may include, for example, an act of operatingthe heat pump in a defrost mode during a defrost cycle includingdelivering refrigerant from the compressor to the first connection pointof the first heat exchanger. Such a method also includes, in variousembodiments, acts of, during the defrost cycle, passing the refrigerantthrough the first heat exchanger from the first connection point, (e.g.,through the multiple cross tubes) to the second connection point of thefirst heat exchanger, and (i.e., also during the defrost cycle), passingthe refrigerant from the second connection point of the first heatexchanger, through the at least one expansion device, and then to thesecond heat exchanger. Such a method also includes, in a number ofembodiments, acts of, during the defrost cycle, passing the refrigerantthrough the second heat exchanger, and then back to the compressor, andduring at least part of the defrost cycle, delivering refrigerant fromthe compressor to the third connection point of the first heat exchangerand passing the refrigerant from the third connection point, (e.g.,through the second header), to the second connection point.

In some such methods, the second header of the first heat exchangerincludes a first end and a second end, each of the cross tubes connectto the second header between the first end and the second end, thesecond connection point of the first heat exchanger is at the second endof the second header, the third connection point of the first heatexchanger is at the first end of the second header, or a combinationthereof. Moreover, in particular embodiments, the act of passing therefrigerant from the third connection point, through the second header,for example, to the second connection point includes passing therefrigerant from the first end, through the second header, to the secondend. Furthermore, in some embodiments, each cross tube includes multiple(e.g., contiguous, parallel, or both) refrigerant passagewaystherethrough, the first heat exchanger further includes multiple fins(e.g., that are between the cross tubes, that are bonded to the crosstubes, or both), and the act of passing the refrigerant through thefirst heat exchanger (e.g., from the first connection point, through themultiple cross tubes, to the second connection point of the first heatexchanger) includes heating the multiple fins between the cross tubes.

In a number of embodiments, the act of delivering refrigerant from thecompressor to the third connection point of the first heat exchangerincludes opening a solenoid valve, for example, in a bypass refrigerantline, for instance, extending from a supply refrigerant line connectedto the first connection point. In particular such embodiments, thebypass refrigerant line extends, for example, to the third connectionpoint. Moreover, in certain embodiments, during a first portion of thedefrost cycle, the method includes not passing refrigerant through thethird connection point, and during a second portion of the defrostcycle, passing refrigerant through the third connection point, forexample. Furthermore, in particular embodiments, the heat pump furtherincludes a first fan that moves air through the first heat exchanger,and the method further includes an act of operating the first fan in areversed direction during at least part of the defrost cycle to reducenatural convection through the first heat exchanger.

In still another specific embodiment, the invention provides a method ofimproving the effectiveness of a defrost cycle of a heat pump, inparticular, by operating the outdoor air fan in a reversed direction toreduce natural convection through the outdoor heat exchanger. In anumber of such embodiments, the heat pump includes an outdoor heatexchanger, a compressor, at least one expansion device, and an indoorheat exchanger, for example. Further, in various embodiments, the methodincludes (e.g., in any order except where a particular order isexplicitly indicated), at least the acts of operating the heat pump in adefrost mode during a defrost cycle wherein refrigerant is deliveredfrom the compressor to the outdoor heat exchanger, and during thedefrost cycle, passing the refrigerant through the outdoor heatexchanger. A number of such methods further include, during the defrostcycle, passing the refrigerant from the outdoor heat exchanger, throughthe at least one expansion device, and then to the indoor heatexchanger, and also during the defrost cycle, passing the refrigerantthrough the indoor heat exchanger, and then back to the compressor. Suchmethods further include, in various embodiments, during at least part ofthe defrost cycle, the act of operating the outdoor air fan in areversed direction to reduce natural convection through the outdoor heatexchanger.

In addition, various other embodiments of the invention are alsodescribed herein, and other benefits of certain embodiments may beapparent to a person of ordinary skill in the art.

DETAILED DESCRIPTION OF EXAMPLES OF EMBODIMENTS

A number of embodiments of the subject matter described herein includeheat pumps with improved defrost cycles, methods of defrosting heatexchangers, and methods of improving the effectiveness of defrost cyclesof heat pumps, as examples. These systems and methods may be used, forinstance, with heat pumps having a microchannel (e.g., outdoor) heatexchanger. Various embodiments include a defrost valve located in arefrigerant conduit that opens during a defrost cycle to deliver hotrefrigerant gas to a portion of the heat exchanger that otherwisedefrosts more slowly or less completely than other portions of the heatexchanger. Particular embodiments pass hot refrigerant gas through aheader of the heat exchanger. Further, in some embodiments, the fan thatis used to blow air (e.g., outdoor air) through the heat exchanger isoperated in a reversed direction during at least part of the defrostcycle to counteract natural convection through the heat exchanger.

In various embodiments, in a defrost mode, during a defrost cycle,refrigerant is delivered from the compressor to a first connection pointof a first heat exchanger. The refrigerant is passed through the firstheat exchanger from the first connection point (e.g., through multiplecross tubes) to a second connection point of the first heat exchanger.Further, also during the defrost cycle, the refrigerant is passed fromthe second connection point of the first heat exchanger, through atleast one expansion device, and then to a second heat exchanger. Evenfurther, the refrigerant is passed, in a number of embodiments, throughthe second heat exchanger, and then back to the compressor. Moreover, incertain embodiments, during at least part of the defrost cycle,refrigerant is delivered from the compressor to a third connection pointof the first heat exchanger and is passed from the third connectionpoint, through the second header, to the second connection point. In anumber of embodiments, the defrost valve is open only during a portionof the defrost cycle.

FIG. 1 illustrates an example of a heat pump having an improved defrostcycle. In this example, heat pump 10 includes first heat exchanger 11,compressor 13, expansion devices 14 and 17, first refrigerant conduit101, second refrigerant conduit 102, third refrigerant conduit 103,defrost valve 15, and control system 16, for example, that controlsdefrost valve 15. Heat exchanger 11 can be a microchannel heatexchanger, for example, or in other embodiments, can be a different typeof heat exchanger. In HVAC applications, for instance, heat exchanger 11can be an outdoor heat exchanger. Further, in the embodiment shown,first heat exchanger 11 includes first connection point 111, secondconnection point 112, and third connection point 113. As used herein,“connection points”, are locations where a refrigerant conduit, such asrefrigerant tubing, connects to the heat exchanger to deliverrefrigerant to or from the heat exchanger. Connection points areopenings on the heat exchanger before the heat exchanger is connected tothe refrigerant conduits.

A refrigerant conduit, as used herein, is an enclosed passageway thatrefrigerant flows through during at least one mode of operation of theheat pump. Refrigerant conduit may include, as examples, tubing (e.g.,copper), pipe, fittings, passageways through valve bodies, passagewaysthrough other components such as mufflers, dryers, accumulators, andcompensators, as examples, or a combination thereof. In the embodimentdepicted, first refrigerant conduit 101 connects discharge port 131 oncompressor 13 to first connection point 111 of first heat exchanger 11.As used herein, in this context, “connects” or “connecting” meansprovides, or providing an enclosed passageway therebetween forrefrigerant to flow through, at least during one mode of operation ofthe heat pump. Further still, in this embodiment, second refrigerantconduit 102 connects second connection point 112 of first heat exchanger11 to expansion devices 14 and 17, and third refrigerant conduit 103connects first refrigerant conduit 101 to third connection point 113 offirst heat exchanger 11.

Moreover, in this particular embodiment, defrost valve 15 is located inthird refrigerant conduit 103, between first refrigerant conduit 101 andthird connection point 113 of first heat exchanger 11. In thisparticular embodiment, when defrost valve 15 is closed, refrigerant flowthrough third refrigerant conduit 103 is blocked (i.e., completelyblocked or substantially blocked to the extent that any leakage has anegligible impact on the performance of the heat pump) at defrost valve15. In the example of heat pump 10, control system 16 opens defrostvalve 15 during the defrost cycle, for example, allowing refrigerant toflow through defrost valve 15 and third refrigerant conduit 103. In thisoperation, refrigerant flows to third connection point 113 to defrostfirst heat exchanger 11 between third connection point 113 and secondconnection point 112, for instance, during at least part of the defrostcycle.

Various embodiments have at least one expansion device, for instance,one or two expansion devices. In the embodiment shown, heat pump 10 hastwo expansion devices 14 and 17. In the embodiment illustrated,expansion device 14 is used when the refrigerant flows in one direction,and expansion device 17 is used when the refrigerant flows in theopposite direction. In this embodiment, expansion device 14 is used whenheat pump 10 is operated in a cooling mode (i.e., cooling second heatexchanger 12) or in a defrost mode (i.e., defrosting first heatexchanger 11) and expansion device 17 is used when heat pump 10 is beingoperated in a heating mode (i.e., heating second heat exchanger 12). Inthis context, an expansion device being “used” means that the expansiondevice produces a substantial restriction to flow or pressuredifferential (i.e., across the expansion device). When an expansiondevice is not being used, refrigerant passes through the expansiondevice or through a check valve arranged in parallel thereto, withlittle or no resistance to flow or pressure differential across theexpansion device.

Although FIG. 1 is not drawn to scale, expansion device 17 wouldtypically be close to first heat exchanger 11, and expansion device 14would typically be close to second heat exchanger 12 (e.g., in a splitHVAC system). Heat pumps that are used to provide heating only, and heatpumps that are used to provide cooling only, typically have just oneexpansion device. Certain heat pumps that both heat and cool, however,may also have just one expansion device, in contrast with heat pump 10illustrated (e.g., packaged units where one expansion device is adequatefor flow in both directions in heating and cooling modes). Examples ofexpansion devices include orifices, orifice tubes, and various types ofexpansion valves. In different embodiments, expansion devices maycontrol superheat (e.g., hold a constant superheat) in the heatexchanger acting as the evaporator. Examples include thermal expansionvalves and electronic expansion valves.

Still referring to FIG. 1, in a number of embodiments, the portion ofthe heat exchanger (e.g., 11) that extends from the at least one firstconnection point (e.g., 111) to the second connection point (e.g., 112)includes at least half of the heat exchanger, for example, in terms ofvolume of the heat exchanger, surface area, or length of the flowpassage through the heat exchanger. Further, in various embodiments, theportion of the heat exchanger (e.g., 11) that extends from the thirdconnection point (e.g., 113) to the second connection point (e.g., 112)includes no more than half of the heat exchanger. Other embodiments,however, may differ.

FIG. 2 illustrates first heat exchanger 20. In certain embodiments, heatexchanger 20 can be substituted for heat exchanger 11 of heat pump 10shown in FIG. 1. Heat exchanger 20, however, has two first connectionpoints 2111 and 2112, which correspond to first connection point 111shown in FIG. 1. As illustrated, first heat exchanger 20 includes firstheader 21, second header 22, and multiple cross tubes 23 extending fromfirst header 21 to second header 22. In this particular embodiment, eachof multiple cross tubes 23 connects to first header 21 and each ofmultiple cross tubes 23 connects to second header 22. As used herein, inthis context, “connects” means that a refrigerant passageway exists fromthe interior of the first header, through the (each) cross tube, to theinterior of the second header.

Moreover, in this embodiment, first header 21 is parallel to secondheader 22, and multiple cross 23 tubes are parallel to each other andperpendicular to first header 21 and to second header 22. As usedherein, “parallel”, in this context, means parallel to within 5 degrees,and “perpendicular”, in this context, means perpendicular to within 5degrees. Further, as used herein, “substantially parallel”, meansparallel to within 10 degrees, and “substantially perpendicular”, meansperpendicular to within 10 degrees. In some embodiments, first header 21is substantially parallel to second header 22, multiple cross 23 tubesare substantially parallel to each other, multiple cross tubes 23 aresubstantially perpendicular to first header 21, multiple cross 23 tubesare substantially perpendicular to second header 22, or a combinationthereof, as other examples.

FIG. 3 is a detailed view showing part of heat exchanger 20. As can beeseen in FIG. 3, multiple cross tubes 23 each include multiple contiguousparallel refrigerant passageways 33 therethrough (i.e., extending fromfirst header 21 to second header 22 shown in FIG. 2). In this particularembodiment, passageways 33 are arranged in a row. In variousembodiments, refrigerant passageways may be arranged in one row, in atleast one row, or in two rows, as examples. Although not visible fromthe angle of FIG. 3, passageways 33 are open into second header 22 atthe bottoms of cross tubes 23. Further, cross tubes 23 are sealedagainst slots in second header 22 where cross tubes 23 pass through thewall of second header 22 to prevent refrigerant from leaking out to theatmosphere. Further still, in this embodiment, first heat exchanger 20further includes multiple fins 34 between cross tubes 23. The fins 34can be bonded to the cross tubes 23, for example, by brazing. In anumber of embodiments, brazing can also seal cross tubes 23 to secondheader 22 (and similarly to first header 21 shown in FIG. 2). In theembodiment illustrated, fins 34 are slanted, which may help to shedcondensation, reduce airflow resistance through the heat pump, improveheat transfer, or a combination thereof, as examples. See U.S. patentapplication Ser. No. 12/561,178, Publication 2010/0071868, for example.In other embodiments, however, the fins may be square (e.g., horizontalor perpendicular to headers 21 and 22 and perpendicular to cross tubes23 or passageways 33) rather than being slanted. Although not shown inFIG. 3, in certain embodiments, fins may include enhancements, such aslouvers, which may improve heat transfer characteristics. Again, seeU.S. patent application Ser. No. 12/561,178, Publication 2010/0071868,for example.

FIG. 4 illustrates that in particular embodiments, the heat pump (e.g.,10 shown in FIG. 1) can further include extension tube 44, for instance,located within second header 42. Heat Exchanger 40 may be similar toheat exchanger 20 shown in FIG. 2, or to heat exchanger 11 shown in FIG.1, except for extension tube 44, and that heat exchanger 40 dischargesfrom second header 42 at both ends 472 and 471 (i.e., at secondconnection points 4121 and 4122 respectively) rather than at just oneend (e.g., end 272 for heat exchanger 20 shown in FIG. 2). In certainembodiments, heat exchanger 40 can be substituted for heat exchanger 11of heat pump 10 shown in FIG. 1. In this embodiment, extension tube 44within second header 42 is substantially parallel to second header 42.As mentioned, as used herein, “substantially parallel” means parallel towithin 10 degrees. In particular embodiments, extension tube 44 may beparallel or concentric with second header 42, for example, or both.

Extension tube 44, in this embodiment, discharges hot refrigerant fromthe compressor (e.g., 13) substantially at midpoint 425 of second header42. As used herein, “substantially at”, in this context, means within 20percent of the length of second header 42 from the exact midpoint ofheader 42. Further, as used herein, “at” a midpoint, in this context,means within 10 percent of the length of second header 42 from the exactmidpoint of header 42. In some embodiments, extension tube 44 dischargeshot refrigerant from the compressor (e.g., 13) at midpoint 425 of secondheader 42. The hot refrigerant then travels from midpoint 425(approximately) to second connection points 4121 and 4122 heating anddefrosting second header 42 and the portion of heat exchanger 40 nearsecond header 42. In this embodiment, the at least one first connectionpoint 411 (e.g., corresponding to first connection point 111 in FIG. 1)to first heat exchanger 40 (e.g., corresponding to heat exchanger 11 inFIG. 1) is at first header 41, the second connection points 4121 and4122 (e.g., corresponding to second connection point 112 in FIG. 1) tofirst heat exchanger 40 are at second header 42, and the thirdconnection point 413 (e.g., corresponding to third connection point 113in FIG. 1) to the first heat exchanger 40 is at extension tube 44. Otherembodiments may deliver hot refrigerant to the midpoint of the (e.g.,second) header in a manner other than through extension tube 44, forexample, through a tee or other fitting or connection in the (e.g.,second) header (e.g., at or substantially at midpoint 425), as otherexamples.

Some embodiments include a distributor tube in the second header (e.g.,second header 22 shown in FIG. 2). The distributor tube may connect tosecond end 272 of header 22, for example, and connection point 212 maybe on or to the distributor tube such that refrigerant conduit 102connects to the distributor tube. The distributor tube may extend intosecond header 22 over a majority of the length of header 22, forexample, and may include holes therethrough. The distributor tube maydistribute refrigerant more evenly to the heat exchanger, for example,when the heat pump is being operated in a heating mode. The distributortube does not necessarily provide any benefit during the cooling mode orthe defrost cycle, but various embodiments of defrost cycle improvementsmay work satisfactorily with a distributor tube. Other embodiments,however, may lack a distributor tube (e.g., as shown in FIG. 2).

As mentioned, in the embodiment illustrated in FIG. 1, heat pump 10 hasjust one first connection point 111. Various embodiments include atleast one first connection point. Particular embodiments describedherein include heat pump 10 with one connection point 111 (e.g., shownin FIG. 1) or with two connection points (e.g., 2111 and 2112 shown inFIG. 2), as examples. FIG. 4 also illustrates an embodiment with onefirst connection point (e.g., 411). Other embodiments may have 3, 4, 5,6, or another number of first connection points, as other examples.Further, other embodiments may have 1, 2, 3, 4, 5, 6, or another numberof third connection points (e.g., 113, 213, or 413), as furtherexamples.

In a number of embodiments, the at least one first connection point(e.g., 111 shown in FIGS. 1, 2111 and 2112 shown in FIG. 2, or 411 shownin FIG. 4) to the first heat exchanger (e.g., 11 shown in FIG. 1, 20shown in FIG. 2, or 40 shown in FIG. 4) is at the first header (e.g., 21shown in FIG. 2 or 41 shown in FIG. 4). As used herein, a connectionpoint being “at” a particular component (e.g., a particular header)means that the connection point is connected to the component such thatrefrigerant that passes through the connection point goes into or out ofthe component. Further, in various embodiments, the second connectionpoint (e.g., 112 shown in FIG. 1, 212 shown in FIG. 2, or 4121 and 4122shown in FIG. 4) to the first heat exchanger (e.g., 11, 20, or 40) is atthe second header (e.g., 22 or 42). Even further, in a number ofembodiments, the third connection point (e.g., 113 shown in FIG. 1, 213shown in FIG. 2, or 413 shown in FIG. 4) to the first heat exchanger(e.g., 11, 20, or 40) is at the second header (e.g., 22 or 42).

Moreover, in the embodiments illustrated in FIGS. 2 and 4, the secondheader (e.g., 22 or 42) has a first end (e.g., 271 or 471) and a secondend (e.g., 272 or 472). As shown in FIG. 2, in this embodiment, each ofthe multiple cross tubes 23 connects to the second header 22 between thefirst end 271 and the second end 272. As used herein, cross tubes areonly considered to be “cross tubes” if they are connected to carryrefrigerant therethrough. Some embodiments of heat exchangers may havethe physical structure of a cross tube (e.g., bonded to the fins) thatis not connected (e.g., to headers 21 and 22) to carry refrigerant. Sucha physical structure, not connected to carry refrigerant, is notconsidered to be a “cross tube” as used herein. In the embodiment shownin FIG. 2, second connection point 212 to first heat exchanger 20 is atsecond end 272 of second header 22. In addition, in this embodiment,third connection point 213 to first heat exchanger 20 is at first end271 of second header 22. Moreover, heat exchanger 20 can be substitutedfor first heat exchanger 11 in FIG. 1. In a number of such embodiments,when defrost valve 15 is open during a defrost cycle, hot refrigerantgas flows through second header 22 defrosting heat exchanger 20 at andnear second header 22.

Other embodiments can have connection points that are not at headers orthat are at headers but are not at the ends of the headers. In someembodiments, for example, one or more connections may tee into theheader at the midpoint (e.g., 425) or spaced along the header, asexamples. In certain embodiments, one or more headers may extend all theway around the unit and may lack an “end”, but may include a tee orother fitting forming a connection point to the header at one or morelocations around the unit. Further, other embodiments of heat exchangersdo not have a header or headers with cross tubes extending between theheaders, but rather, have a continuous refrigerant pathway that may belarger in cross section than the passageways of the cross tubesdescribed herein and longer, in order to provide the necessary ordesired heat transfer performance.

Furthermore, in the embodiment shown in FIG. 2, first header 21 hasthird end 273 and fourth end 274. In this embodiment, each of themultiple cross tubes 23 connects to first header 21 between third end273 and fourth end 274. In addition, in this particular embodiment, theat least one first connection point (e.g., corresponding to firstconnection point 111 shown in FIG. 1) to the first heat exchanger (e.g.,11 shown in FIG. 1 or 20 shown in FIG. 2) comprises or includes primaryfirst connection point 2111 to heat exchanger 20 at third end 273 offirst header 21 and secondary first connection point 2112 to heatexchanger 20 at fourth end 274 of first header 21. In certainembodiments, for instance, primary first connection point 2111 andsecondary first connection point 2112 may be of equal size, identicalbut opposite hand, or both, as examples. In other embodiments, however,primary first connection point 2111 and secondary first connection point2112 may differ in terms of size, configuration, or both.

Moreover, in the embodiment shown, if heat exchanger 20 is substitutedfor first heat exchanger 11 shown in FIG. 1, first refrigerant conduit201 shown in FIG. 2, which is analogous to first refrigerant conduit 101shown in FIG. 1, connects the discharge port (e.g., 131 shown in FIG. 1)on the compressor (13 shown in FIG. 1) to primary first connection point2111 and to secondary first connection point 2112. In other embodiments,the at least one first connection point (e.g., corresponding to 111shown in FIG. 1) to the first heat exchanger (e.g., 11 shown in FIG. 1or 21 shown in FIG. 2) consists of a single first connection point(e.g., 2111) at the third end (e.g., 273) of the first header (e.g.,21), as another example. As used herein, a particular connection point“consisting of a single connection point” (or that “consists of a singleconnection point”) means that the particular connection point includesonly one connection point (i.e., rather than two or more connectionpoints). FIG. 4 illustrates an example wherein the at least one firstconnection point (e.g., corresponding to 111 shown in FIG. 1) to thefirst heat exchanger (e.g., 40) consists of single first connectionpoint 411 at third end 473 of first header 41. In this particularembodiment of heat exchanger 40, fourth end 474 may be plugged orcapped, for example, and may not be used (i.e., as a connection point).

Further, in yet another embodiment, the first refrigerant conduit (e.g.,101 shown in FIG. 1) can connect the discharge port (e.g., 131) on thecompressor (e.g., 13 shown in FIG. 1) to the fourth end (e.g., 274 or474) of the first heat exchanger (e.g., 20 or 40) and a furtherrefrigerant conduit or portion of a refrigerant conduit, may connect thethird end (e.g., 273 or 473) of the first header (e.g., 21 or 41) to thefirst end (e.g., 271 or 471) of the second header (e.g., 22 or 42). Inthis particular embodiment, the defrost valve (e.g., 15) can be locatedin this further refrigerant conduit or portion of a refrigerant conduitbetween the third end (e.g., 273 or 473) of the first header (e.g., 21or 41) and the first end (e.g., 271 or 471) of the second header (e.g.,22 or 42). Referring to FIG. 1, in this embodiment, as used herein, thefirst header (21 or 41) can be considered part of the first refrigerantconduit (e.g., 101) or part of the third refrigerant conduit (e.g.,103).

In various embodiments, the control system (e.g., 16 shown in FIG. 1)includes a digital controller (e.g., 160) that includes programminginstructions (e.g., 161) to open the defrost valve (e.g., 15), forinstance, during the defrost cycle, to defrost the first heat exchanger(e.g., 11, 20, or 40), for example, between the third connection point(e.g., 113, 213, or 413) and the second connection point (e.g., 112,212, or 4121 and 4122). Digital controller 160 may include, forinstance, a microprocessor, memory, software, a display, a keyboard, atouch screen, electrical connectors, or a combination thereof, asexamples. In some embodiments, digital controller 160 can be part of adefrost control board, for example, that may be located in an outdoorunit or in a packaged system, as examples. Digital controller 160 mayopen a valve, for example, (e.g., defrost valve 15) by sending a signalto the valve, by directing power to the valve, or by sending a signal toa relay to send power to the valve, as examples. Control system 16 ordigital controller 160 may control various components of the heat pump(e.g., 10). Certain control pathways are illustrated with broken linesin FIG. 1. The illustrated control pathways, however, are notnecessarily exhaustive. Other components of the heat pump may becontrolled by control system 16 or digital controller 160 as well.Control pathways may include power wires, 24 V AC control wiring,digital signals, or a combination thereof, as examples. In someembodiments, control signals may be sent through wireless communicationsor via signals sent over power wires, as other examples. Control system16 or digital controller 160 may turn various electrical components onand off, may control speeds of various motors (e.g., fan motors, thecompressor motor, or a combination thereof), may control temperature(e.g., space temperature), may control defrost cycles, or a combinationthereof, as examples.

Furthermore, in some embodiments, the digital controller (e.g., 160)further includes programming instructions (e.g., 162) to keep thedefrost valve (e.g., 15) closed when the heat pump (e.g., 10) is not inthe defrost cycle. Keeping the defrost valve closed, as used herein,means while the heat pump (e.g., compressor 13) is operating. When theheat pump is not operating (e.g., when compressor 13 is stopped or off),the defrost valve (e.g., 15) can be closed or open. In a number ofembodiments, however, defrost valve 15 is normally closed, is closedwhen not powered, or is closed when the heat pump (e.g., compressor 13)is stopped or off, for instance. In addition, in some embodiments, thedigital controller (e.g., 160) further includes programming instructions(e.g., 163) to keep the defrost valve (e.g., 15) closed during part ofthe defrost cycle, for example, to direct more of the hot refrigerantthrough the at least one first connection point (e.g., 111, 2111 and2112, or 411) to defrost, or to better defrost, the first heat exchanger(e.g., 11, 20, or 40) between the at least one first connection point(e.g., 111, 2111 and 2112, or 411) and the second connection point(e.g., 112, 212, or 4121 and 4122).

Furthermore, heat pump 10, in the embodiment shown in FIG. 1, furtherincludes first fan 18 positioned and configured, for example, as shown,to move air (e.g., outdoor air or outside air) through first heatexchanger 11. In certain embodiments, the control system (e.g., 16) ordigital controller (e.g., 160) includes programming instructions (e.g.,164) to operate the first fan (e.g., 18) in a reversed direction duringat least part of the defrost cycle, for example, to reduce naturalconvection through the first heat exchanger (e.g., 11) during the atleast part of the defrost cycle. As used herein, in this context,“reversed direction” means rotating in the opposite direction from fanoperation in the heating or cooling mode. In various embodiments, thefan may be operated, in the defrost cycle or in part of the defrostcycle, in a direction that blows air downward, to counteract naturalconvection that tends to move the warm air upward. Reversed operation ofthe first fan or outdoor fan can benefit heat pumps with a microchannelheat exchanger (e.g., as shown in FIGS. 2 and 3), for example, but indifferent embodiments, can also benefit heat pumps with other types ofheat exchangers.

In a number of embodiments, the first fan (e.g., 18) may be operated ata reduced or substantially reduced rate of speed (e.g., in the reverseddirection) during the defrost cycle, in comparison with operation in theheating or cooling mode. As used herein, a “substantially reduced rateof speed” is less than or equal to 25 percent of the rated or maximumrate of speed. In particular embodiments, however, the “substantiallyreduced rate of speed” can be 25, 20, 15, 12, 10, 8, 7, 6, 5, 4, 3, 2,1, or ½ percent of the rated or maximum rate of speed (e.g., of fan 18,fan motor 180, or a drive system therefor), as examples. Moreover, incertain embodiments, the “substantially reduced rate of speed” can beaccomplished by intermittent operation (e.g., intermittent powering) orpulsing of the electrical power to the fan motor (e.g., 180). Inparticular embodiments, this intermittent operation or pulsing of thefan motor (e.g., 180) can be controlled by control system 16 or digitalcontroller 160, for example, in the defrost board.

In a number of embodiments, motor 180 is a variable-speed motor and iscapable of running in the reversed direction at a low speed. In variousembodiments, a variable-speed drive unit may be included, such as avariable-frequency AC drive unit or a variable-voltage DC drive unit, asexamples. In some embodiments, the minimum speed provided by thevariable-speed drive unit may be sufficiently low with steady electricalpower being provided to the motor (e.g., 180). In certain embodiments,however, the speed can be lowered further by providing power to themotor (e.g., 180) intermittently. In particular embodiments, thisintermittent operation or pulsing of the fan motor (e.g., 180) can becontrolled by control system 16 or digital controller 160, for example,by controlling the variable-speed drive unit.

Further, in some embodiments, the fan motor (e.g., 180) may be asingle-speed motor or a two-speed motor, as examples, a variable-speeddrive unit may not be provided, or a combination thereof, and the“substantially reduced rate of speed” may be accomplished byintermittent operation (e.g., intermittent powering) or pulsing of theelectrical power to the fan motor (e.g., 180). In particularembodiments, this intermittent operation or pulsing of the fan motor(e.g., 180) can be controlled by control system 16 or digital controller160, for example, by actuating a relay that turns electrical power tothe fan motor (e.g., 180) on and off. Further, reversed operation of thefirst fan or outdoor fan can be used in combination with a defrostvalve, or on units that do not have a defrost valve.

As further illustrated in FIG. 1, in some embodiments, the heat pump(e.g., 10) further includes a reversing valve (e.g., 150) located, forexample, in the first refrigerant conduit (e.g., 101) between thedischarge port (e.g., 131) on the compressor (e.g., 13) and the at leastone first connection point (e.g., 111) of the first heat exchanger(e.g., 11). The reversing valve (e.g., 150) can be used to switch theheat pump between heating and cooling modes or between heating anddefrost modes, for example. In the particular embodiment shown, forinstance, third refrigerant conduit 103 connects to first refrigerantconduit 101 between reversing valve 150 and (e.g., the at least one)first connection point 111 of first heat exchanger 11. Moreover, in theembodiment show, heat pump 10 further includes second heat exchanger 12,a fourth refrigerant conduit 104 connecting (e.g., the at least one)expansion device 14 to second heat exchanger 12. Moreover, heat pump 10further includes fifth refrigerant conduit 105 connecting second heatexchanger 12 to reversing valve 150, and sixth refrigerant conduit 106connecting reversing valve 150 to inlet port 132 on compressor 13. Inthis particular embodiment, accumulator 170 is provided in, or connectedto, sixth refrigerant conduit 106. Reversing valve 150 can be controlledby control system 16 or digital controller 160, for example.

In the embodiment illustrated, defrost valve 15 is a separate valve.Defrost valve 15 can be a solenoid valve, for example, that is eitherfully open or fully closed. Defrost valve 15 can be electricallyoperated, pilot operated, or both, as examples. In some embodiments, acheck valve can be provided in series with defrost valve 15 to allowflow only in one direction, while in other embodiments, defrost valve 15can be kept closed when flow through defrost valve 15, in eitherdirection, is undesirable. Further, in the embodiment illustrated, whendefrost valve 15 is open, refrigerant from compressor 13 can flowthrough defrost valve 15 to third connection 113, 213, or 413, andthrough first connection 111, 2111 and 2112, or 411. In otherembodiments, however, a three way valve can be used, or two of thetwo-way valves can be used, so that when refrigerant from the compressoris directed to the third connection point, the refrigerant is preventedfrom also flowing to the first connection point.

In the embodiment shown, defrost valve 15, refrigerant conduit 103, orboth, can be sized to deliver an appropriate amount of hot refrigerantto third connection 113, 213, or 413, for instance. Further, in someembodiments, defrost valve 15, refrigerant conduit 103, or both, can besized so that less than half of the refrigerant from compressor 13passes through third connection 113, 213, or 413, for instance, whilemore than half of the refrigerant from compressor 13 passes throughfirst connection 111, 411, or 2111 and 2112. In other embodiments,defrost valve 15 can be of a type suitable to modulate and throttlerefrigerant therethrough and can deliver a regulated or measured amountof refrigerant to third connection 113, 213, or 413, as another example.In some embodiments, defrost valve 15 can be part of an integrated valvemodule that performs other functions as well, or can be part of anothercomponent. In particular embodiments, for example, the defrost valve canbe part of the reversing valve (e.g., 150), for example.

In many of the embodiments described herein, during a defrost cycle,refrigerant is delivered to the (e.g., first) heat exchanger (e.g., 11)at two different connection points (e.g., 111 and 113) and is removedfrom the heat exchanger at one connection point (e.g., 112). Delivery ofrefrigerant to one of the two connection points (e.g., 113) is turned onand off, and is off in the typical heating or cooling modes (e.g., usingdefrost valve 15). In other embodiments, during a defrost cycle,refrigerant is delivered to the heat exchanger at one connection pointand is removed from the heat exchanger at two different connectionpoints. In still other embodiments, during a defrost cycle, refrigerantis delivered to the heat exchanger at three different connection points(e.g., 2111, 2112, and 213) and is removed from the heat exchanger atone connection point (e.g., 212). Delivery of refrigerant to one of thethree connection points (e.g., 213) is turned on and off, and is off inthe typical heating or cooling modes (e.g., using defrost valve 15). Infurther embodiments, during a defrost cycle, refrigerant is delivered tothe heat exchanger at two different connection points (e.g., 411 and413) and is removed from the heat exchanger at two different connectionpoints (e.g., 4121 and 4122). Delivery of refrigerant to one of the twoconnection points (e.g., 413) is turned on and off, and is off in thetypical heating or cooling modes (e.g., using defrost valve 15). Instill other embodiments, refrigerant may be passed through a heatexchanger first in one direction, and then in an opposite direction, topromote more-even defrosting of the heat exchanger, as another example.

FIG. 5 illustrates outdoor unit 50 for a split system HVAC system, witha corner access panel removed and a louvered cover removed from over the(first) heat exchanger. As used herein, an HVAC system is a system thatprovides air conditioning, heating, or both air conditioning andheating, as well as providing air movement (ventilation). Further, asused herein, an HVAC heat pump provides heating to the space (e.g.,within the building), for example, using the compressor. Unit 50includes a first heat exchanger, which may be similar to heat exchanger20 shown in FIG. 2. Unit 50 also includes, in this particularembodiment, components of heat pump 10 shown in FIG. 1, except thatsecond heat exchanger 12, second fan 19, expansion device 14,refrigerant conduit 104, and part of refrigerant conduits 102 and 105are located in a separate indoor unit or air handler, which may alsoinclude an air filter, among other things. Control system 16, digitalcontroller 160, or both, may be located in unit 50, in the indoor unit,elsewhere, or a combination thereof.

Unit 50 has two first connection points 2111 and 2112 to first heatexchanger 20, as shown in FIG. 2, rather than a single first connectionpoint 111 shown in FIG. 1. Not shown in FIG. 2, the first heat exchangerin unit 50 includes three 90-degree radiused bends, in this embodiment,and extends around unit 50 from first connection point 2111 to secondconnection point 2112. In other embodiments, heat pump 10, for example,may be a packaged HVAC unit, or may be another type of heat pump, asexamples. As shown in FIGS. 2 and 4, in a number of embodiments, thefirst header is at the top of the first heat exchanger and the secondheader is at the bottom of the first heat exchanger, and the cross tubesare vertical. In other embodiments, however, the headers may be inopposite locations, or may be on opposite sides with the cross tubeshorizontal, as other examples.

FIG. 6 illustrates an example of various methods, method 600. Method 600may be, for example, a method of defrosting a first heat exchanger(e.g., 11 shown in FIG. 1, 20 shown in FIG. 2, or 40 shown in FIG. 4) ofa heat pump (e.g., 10 or 50 shown in FIGS. 1 and 5, respectively). Sucha heat pump (e.g., 10 or 50) can include, for example, the first heatexchanger (e.g., 11, 20, or 40), a compressor (e.g., 13), at least oneexpansion device (e.g., 14), and a second heat exchanger (e.g., 12), forinstance. Moreover, in certain embodiments, the first heat exchanger(e.g., 11, 20, or 40) includes (e.g., as shown in FIG. 2) a first header(e.g., 21), a second header (e.g., 22), and multiple cross tubes (e.g.,23) extending from the first header (e.g., 21) to the second header(e.g., 22). In a number of embodiments, for example, each of the crosstubes (e.g., 23) connects to the first header (e.g., 21) and to thesecond header (e.g., 22). In various embodiments, the first heatexchanger (e.g., 11, 20, or 40) includes a first connection point (e.g.,111, 2111, 2112, or 411), a second connection point (e.g., 112, 212,4121, or 4122), and a third connection point (e.g., 113, 213, or 413).Further, in particular embodiments, the first connection point (e.g.,111, 2111, 2112, or 411) is to the first header (e.g., 21 or 41), thesecond connection point (e.g., 112, 212, 4121, or 4122), is to thesecond header (e.g., 22 or 42), and the third connection point (e.g.,113, 213, or 413) is also to the second header (e.g., 22 or 42).

In a number of embodiments, various methods include (e.g., in any orderexcept where a particular order is explicitly indicated), at leastcertain acts. As used herein, “any order” includes acts being performedat the same time. The example of method 600, shown in FIG. 6, startswith act 601 of starting the defrost cycle. In a number of embodiments,the defrost cycle may be started by the control system (e.g., 16 shownin FIG. 1) or, in particular, by digital controller 160. In differentembodiments, controller 160, for instance, may start the defrost cycle(e.g., act 601) based on one or more measured parameters, such as howlong the heat pump (e.g., 10) has been operating, the outdoor airtemperature, temperature of air at the first heat exchanger (e.g., 11,20, or 40), the temperature of the first heat exchanger itself (e.g.,11, 20, or 40), system refrigerant pressure, a reading from a defrostsensor (e.g., 25 shown in FIG. 2), air low through the first heatexchanger (e.g., from sensor 185 shown in FIG. 1), whether (or the rateat which) the indoor air temperature has been changing (e.g.,increasing), the temperature of second heat exchanger 12, or acombination thereof, as examples. Heat pump 10 or 50, or control system16, as examples, may start the defrost cycle (e.g., act 601), forexample, by switching reversing valve 150 from a heating mode to adefrost mode. In a number of embodiments, fan 18 may also be turned offor reversed, which is described in more detail below. The indoor air fanor second fan (e.g., 19) may stay on and the compressor (e.g., 13) maystay on as well. If not already on, indoor air fan or second fan (e.g.,19) and the compressor (e.g., 13) may start. Method 600, for example, inact 601, can include operating the heat pump (e.g., 10 or 50) in adefrost mode during a defrost cycle.

In the embodiment illustrated, method 600 also includes act 602 ofdelivering refrigerant from the compressor (e.g., 13) to the firstconnection point (e.g., 111 shown in FIGS. 1, 2111 and 2112 shown inFIG. 2 and FIG. 5, or 411 shown in FIG. 4) of the first heat exchanger(e.g., 11, 20, or 40). Refrigerant may be delivered (e.g., in act 602),through refrigerant conduit 101 or 201, for example, by operatingcompressor 13. Method 600 also includes, in the embodiment shown, act603 of (e.g., during the defrost cycle started in act 601) passing therefrigerant, for instance, through the first heat exchanger (e.g., 11,20, or 40), from the first connection point (e.g., 111, 2111 and 2112,or 411), to the second connection point (e.g., 112, 212, or 4121 and4122) of the first heat exchanger, (e.g., 11, 20, or 40). Referring toFIG. 2, in act 603, the refrigerant may pass through the multiple crosstubes (e.g., 23), the first header (e.g., 21), the second header (e.g.,22), or a combination thereof, for example.

In this particular embodiment, method 600 also includes act 604 of(e.g., during the defrost cycle), passing the refrigerant from thesecond connection point (e.g., 112 shown in FIG. 1, 212 shown in FIG. 2and FIG. 5, or 4121 and 4122 shown in FIG. 4) of the first heatexchanger (e.g., 11, 20, or 40) to the second heat exchanger (e.g., 12).Referring to FIG. 1, refrigerant may be passed (e.g., in act 604),through refrigerant conduits 102 and 104, for example. In a number ofembodiments, the refrigerant may pass from the second connection point(e.g., 112 shown in FIG. 1) of the first heat exchanger (e.g., 11),through the at least one expansion device (e.g., 17, 14, or both), andthen to the second heat exchanger (e.g., 12). Furthermore, in theembodiment shown, method 600 includes (e.g., during the defrost cyclestarted in act 601) act 605 of passing the refrigerant from the secondheat exchanger (e.g., 12) to the compressor (e.g., 13). Referring toFIG. 1, refrigerant may be passed (e.g., in act 605), throughrefrigerant conduits 105 and 106, for example. In the embodiment shownin FIG. 1, in act 605, the refrigerant also passes through reversingvalve 150 and accumulator 170 between second heat exchanger 12 and inletport 132 on compressor 13. In various embodiments, the refrigerant maypass through the second heat exchanger (e.g., 12), and then back to thecompressor (e.g., 13).

In a number of embodiments, acts 602 to 605 may take place at the sametime. Further, acts 602 to 605 may take place, in some embodiments,starting during act 601, for example, when act 601 begins. Moreover,where refrigerant is described as passing through one component and thenanother component, refrigerant may be passing through both components atthe same time, but the word “then” indicates that the one component islocated upstream from the other component. In the embodimentillustrated, method 600, shown in FIG. 6, also includes act 606 of(e.g., during at least part of the defrost cycle), deliveringrefrigerant from the compressor (e.g., 13) to the third connection point(e.g., 113, 213, or 413) of the first heat exchanger (e.g., 11, 20, or40), and passing the refrigerant from the third connection point (e.g.,113, 213, or 413), to the second connection point (e.g., 112, 212, or4121 and 4122). In some embodiments, the refrigerant may pass, in act606, through the second header (e.g., 22 or 42), for example. Act 606can include, or can be performed by, in the embodiment illustrated,opening defrost valve 15 shown in FIGS. 1, 2, 4, and 5. In theembodiment shown, part of the refrigerant from compressor 13 goesthrough defrost valve 15 and third connection 113, 213, or 413, (andthen through part of the first heat exchanger) and part of therefrigerant from compressor 13 goes through first connection 111, 2111and 2112, or 411, and then though the first heat exchanger (e.g., duringact 606). Thus, in the embodiment illustrated, act 606 is performedduring part (or, in certain embodiments, all) of act 602.

Concerning acts 602 and 606, for example, as used herein, operating in adefrost mode or in a defrost cycle, to defrost the first heat exchanger,requires that the refrigerant be delivered from the compressor (e.g.,13) to the first heat exchanger (e.g., 11, 20, or 40) without therefrigerant passing through the second heat exchanger (e.g., 12) betweenthe compressor and the first heat exchanger. Consequently, therefrigerant is hot when it reaches the first heat exchanger to defrostthe first heat exchanger. The refrigerant can pass through othercomponents, however, between the compressor and the first heatexchanger, such as reversing valve 150 shown in FIG. 1, where someincidental heat transfer and cooling of the hot refrigerant may takeplace. The refrigerant delivered in acts 602 and 606 may be hot,high-pressure gas, in a number of embodiments. Further, in variousembodiments, at least some of the refrigerant gas may give up heat andcondense in the first heat exchanger in act 603 act 606, or both. Therefrigerant then drops from high pressure to low pressure at expansiondevice 14, in the embodiment illustrated, in act 604. This drop fromhigh pressure to low pressure at expansion device 14 in act 604 causesat least some of the refrigerant to change from liquid to vapor (toboil) and become significantly cooler. More of the refrigerant maychange from liquid to vapor (boil) as the refrigerant passes through thesecond heat exchanger 12, for example, absorbing heat from the space,before returning to compressor 13 in act 605.

In various embodiments, the heat pump can include a first fan (e.g., 18shown in FIG. 1) that moves air (e.g., outdoor air) through the firstheat exchanger (e.g., 11, 20, or 40). In the particular embodimentshown, method 600, shown in FIG. 6, also includes act 607 of operatingthe first fan (e.g., 18) in a reversed direction during at least part ofthe defrost cycle, for example, to reduce natural convection through thefirst heat exchanger (e.g., 11, 20, or 40). In various embodiments, thefirst fan can be operated, in the defrost cycle or in part of thedefrost cycle, in a direction that blows air downward, to counteractnatural convection that tends to move the warm air upward. In manyembodiments, this is the reversed direction of rotation, but in otherembodiments, it can be the forward direction of rotation, as anotherexample.

In a number of embodiments, act 607 includes operating the first fan(e.g., 18) at a reduced or substantially reduced rate of speed (e.g., inthe reversed direction) during the defrost cycle, in comparison withoperation in the heating or cooling mode, for example. Moreover, inparticular embodiments, the reduced or substantially reduced rate ofspeed can be accomplished in act 607 by intermittent operation (e.g.,intermittent powering) or pulsing of power to the fan motor (e.g., 180),for instance, under the control of control system 16 or digitalcontroller 160 (e.g., including programming instructions or softwareoperating thereon). Reducing natural convection through the heatexchanger can result in the heat exchanger defrosting more effectively,more quickly, or both, for instance, at least under particularcircumstances. Further, in certain embodiments, the fan (e.g., 18) maybe operated, for instance, briefly, at a high speed in a forward orreversed direction (or both, alternately), for instance, at the end ofthe defrost cycle, to blow moisture, debris, or both from the heatexchanger or to dry the heat exchanger. Other embodiments, however, mayomit this act of high-speed fan operation in the defrost cycle.

In different embodiments, acts 606, 607, or both, can be performedduring all or part of the defrost cycle. For example, in certainembodiments, during a first portion of the defrost cycle, therefrigerant is not delivered to or passed through the third connectionpoint (e.g., 113, 213, or 413). In other words, in this first portion ofthe defrost cycle, act 606 is not performed. The defrost valve (e.g.,15), for example, may remain closed during this first portion of thedefrost cycle. Then during a second portion of the defrost cycle, thisexample of the method includes delivering and passing refrigerantthrough the third connection point (e.g., 113, 213, or 413), forinstance, by opening the defrost valve (e.g., 15). Thus, act 606 isperformed during the second portion of the defrost cycle, in thisembodiment. During these different portions of the defrost cycle,different portions of the heat exchanger (e.g., 11, 20, or 40) aredefrosted or defrosting is focused in those portions of the heatexchanger during these portions of the defrost cycle. In particular, inthis example, in the case of heat exchanger 20 shown in FIG. 2, duringthe first portion of the defrost cycle, first header 21 and most of thelength of cross tubes 23 are defrosted. Then during the second portionof the defrost cycle, second header 22 and the bottom ends of crosstubes 22 are defrosted or are defrosted more effectively. In otherembodiments, the different portions of the heat exchanger may bedefrosted in a different order, at the same time, or by alternatingbetween a greater number of portions of the defrost cycle, as examples.

Furthermore, in different embodiments, the fan (e.g., 18) can beoperated in the reversed direction (i.e., reversed in comparison withfan operation in the heating mode or the cooling mode) during all orpart of the first portion of the defrost cycle, during all or part ofthe second portion of the defrost cycle, or both. In other words, insome embodiments, act 607 is performed during just part of the defrostcycle (e.g., started in act 601). In a number of embodiments, when thefan (e.g., 18) is not being operated in the reversed direction, the fancan be turned off. The speed of the fan (e.g., 18) in the reverseddirection and the extent to which it is operated in the reverseddirection, as opposed to being turned off, can be experimentallydetermined. In addition, the amount of time and sequence that thedefrost valve is open or that hot refrigerant is delivered to the thirdconnection point (e.g., 113, 213, or 413) can be experimentallydetermined. In other embodiments, however, feedback can be utilized tocontrol one or more aspects of the defrost cycle. For example, in someembodiments, feedback from defrost sensor 25 shown in FIG. 2 can be used(e.g., by control system 16) to determine when to end (e.g., in act 608)a defrost cycle, when to open defrost valve 15, or both. In someembodiments, multiple defrost sensors may be provided at differentlocations in the first heat exchanger for such purposes. As anotherexample, in some embodiments, feedback from a flow sensor (e.g., 185shown in FIG. 1) can be used (e.g., by control system 16) to determinewhen (i.e., in the defrost cycle) or whether to operate fan 18 in thereversed direction, the speed to operate fan 18 (i.e., in the reverseddirection) or both. For instance, in some embodiments, control system 16adjusts the speed of first fan 18 to obtain a zero or near zero airflowrate at flow sensor 185 during all or part of the defrost cycle.

Referring to FIGS. 1, 2, and 5, as well as FIG. 6, in variousembodiments, the second header (e.g., 22) of the first heat exchanger(e.g., 20) includes a first end (e.g., 271) and a second end (e.g.,272), each of the cross tubes (e.g., 23) connect to the second header(e.g., 22) between the first end (e.g., 271) and the second end (e.g.,272), the second connection point (e.g., 112 or 212) of the first heatexchanger (e.g., 11 or 20) is at the second end (e.g., 272) of thesecond header (e.g., 22), and the third connection point (e.g., 113 or213) of the first heat exchanger (e.g., 11 or 20) is at the first end(e.g., 271) of the second header (e.g., 22). Moreover, in particularembodiments, act 603, which includes passing the refrigerant from thethird connection point (e.g., 113 or 213), through the second header(e.g., 22), to the second connection point (e.g., 112 or 212), furtherincludes passing the refrigerant from the first end (e.g., 271), throughthe second header (e.g., 22), to the second end (e.g., 272).

Referring to FIGS. 2, 3 and 5, in various methods, each cross tube(e.g., 23) includes multiple (e.g., contiguous, parallel, or both)refrigerant passageways (e.g., 33) therethrough. Moreover, in a numberof embodiments, the first heat exchanger (e.g., 20) further includesmultiple fins (e.g., 34) that can be located between the cross tubes(e.g., 23), that can be bonded to the cross tubes (e.g., 23), or both.Furthermore, in a number of embodiments, act 603 of passing therefrigerant through the first heat exchanger (e.g., 20) from the firstconnection point (e.g., 111 shown in FIG. 1 or 2111 and 2112 shown inFIG. 2), to the second connection point (e.g., 112 or 212) of the firstheat exchanger (e.g., 11 or 20) includes heating the multiple fins(e.g., 34), for instance, between the cross tubes (e.g., 23). The finsmay be heated, for example, to a temperature above freezing (i.e., above32 degrees F. or 0 degrees C.), or to a higher temperature. In differentembodiments, the fins may be heated to a temperature of 35, 40, 45, 50,55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, or 150degrees F., as examples, in act 603. In a number of embodiments, thetemperature of different fins, or different parts of the same fin, mayvary, and in various embodiments, the temperature that the fins reachmay depend on ambient temperature, wind conditions, space temperature,the amount of frost present, other factors, or a combination thereof. Inthe embodiment illustrated, hot refrigerant from the compressor (e.g.,13) is passed (e.g., in act 603) through the multiple cross tubes (e.g.,23). Heat transfers from the refrigerant to the cross tubes (e.g., 23),and then by conduction to the fins (e.g., 34) to melt frost and ice fromthe fins.

In a number of embodiments, act 606, shown in FIG. 6, of deliveringrefrigerant from the compressor (e.g., 13 shown in FIGS. 1 and 5) to thethird connection point (e.g., 113, 213, or 413 shown in FIGS. 1, 2, 4,and 5) of the first heat exchanger (e.g., 11, 20, or 40) includesopening a solenoid valve (e.g., defrost valve 15), for example, in abypass refrigerant line (e.g., conduit 103), for instance, extendingfrom a supply refrigerant line (e.g., conduit 101 or 201) connected tothe first connection point (e.g., 111, 2111 and 2112, or 411). Inparticular such embodiments, the bypass refrigerant line (e.g., 103)extends, for example, to the third connection point (e.g., 113, 213, or413). In different embodiments, the second header (e.g., 22 or 42) maybe heated to a temperature of 35, 40, 45, 50, 55, 60, 65, 70, 75, 80,85, 90, 95, 100, 110, 120, 130, 140, or 150 degrees F., as examples, inact 606. In a number of embodiments, the temperature may depend onambient temperature, wind conditions, space temperature, the amount offrost present, other factors, or a combination thereof. In variousembodiments, hot refrigerant from the compressor (e.g., 13) is passed(e.g., in act 606) through the second header (e.g., 22 or 42). Heattransfers from the refrigerant to the second header (e.g., 22 or 42),and then by conduction to the adjacent (e.g., lower) part of cross tubes23 and to the fins (e.g., 34) bonded thereto to melt frost and ice fromthe second header, adjacent part of the cross tubes, and adjacent fins,for example.

Some methods include just a portion of the acts illustrated in method600 in FIG. 6. Further, some embodiments may include additional acts notshown in FIG. 6. As examples, some methods include acts 601 to 606, butnot act 607, and other methods include acts 601 to 605 and 607, but notact 606 (e.g., methods of improving the effectiveness of a defrost cycleof a heat pump by operating the outdoor air fan in a reversed directionto reduce natural convection through the outdoor heat exchanger). Otherembodiments include all of acts 601 to 607, as another example. Othercombinations and sub-combinations may be apparent to a person ofordinary skill in the art.

Further, various methods include act 608, shown in FIG. 6, of returningto the heating mode. Referring to FIG. 1, as used herein, such a heatingmode (e.g., of act 608) means that the second heat exchanger (e.g., 12)is being heated by the refrigerant. In such a heating mode, as usedherein, the first heat exchanger (e.g., 11) is being cooled by therefrigerant. In a number of HVAC applications, the first heat exchanger(e.g., 11) is located outdoors, or in outdoor air, and in the heatingmode (e.g., returned to in act 608) the first heat exchanger is cooledby the refrigerant to a temperature below that of the outdoor air, andheat is transferred at the first heat exchanger (e.g., 11) from theoutdoor air to the refrigerant. In freezers, however, the first heatexchanger (e.g., 11) can be located in the freezer, or air from thefreezer can be blown through the first heat exchanger (e.g., 11) andreturned to the freezer, and heat can be transferred from the freezer tothe refrigerant, as another example. The heat pump (e.g., 10) may returnto the heating mode (e.g., in act 608) after the defrost cycle iscompleted, for example, after the first heat exchanger (e.g., 11, 20, or40) has been defrosted. In a number of embodiments, method 600 may berepeated when another defrost cycle is needed (e.g., when frost forms onthe first heat exchanger, for example, 11, 20, or 40, for instance, asdetected by frost sensor 25 shown in FIG. 2, or based on a decrease inair flow as determined by airflow sensor 185 shown in FIG. 1) or whenfrost has been deemed to have formed thereon (e.g., based on time ofoperation, one or more temperatures) or a combination thereof.

In a number of embodiments, act 608, of returning to the heating mode,may include, for example, switching reversing valve 150 (i.e., to theheating mode), and operating fan 18 in the normal forward direction.There can be a delay, in some embodiments, before fan 18 is started inthe forward direction, for instance, until heat exchanger 11 becomescold. Compressor 13 and indoor air fan or second fan 19 can continue tooperate (e.g., through act 608), in a number of embodiments. On theother hand, if the thermostat does not call for heating, act 608 ofreturning to the heating mode may include turning off the unit untilheating is demanded by the thermostat. Act 608 may be initiated bycontrol system 16 or digital controller 160, for example.

Various embodiments of the subject matter described herein includevarious combinations of the acts, structure, components, and featuresdescribed herein, shown in the drawings, or known in the art. Moreover,certain procedures may include acts such as obtaining or providingvarious structural components described herein, obtaining or providingcomponents that perform functions described herein. Furthermore, variousembodiments include advertising and selling products that performfunctions described herein, that contain structure described herein, orthat include instructions to perform functions described herein, asexamples. Such products may be obtained or provided throughdistributors, dealers, or over the Internet, for instance. The subjectmatter described herein also includes various means for accomplishingthe various functions or acts described herein or apparent from thestructure and acts described.

1. A heat pump having an improved defrost cycle, the heat pumpcomprising: a compressor; at least one expansion device; a first heatexchanger comprising: a first header; a second header; and multiplecross tubes extending from the first header to the second header,wherein: each of the multiple cross tubes connects to the first header;each of the multiple cross tubes connects to the second header; thefirst header is parallel to the second header; the multiple cross tubesare parallel to each other; and the multiple cross tubes each includemultiple contiguous parallel refrigerant passageways therethrough;multiple fins between the cross tubes wherein the fins are bonded to thecross tubes; at least one first connection point to the first heatexchanger where refrigerant is delivered to the first heat exchangerfrom the compressor during the defrost cycle; a second connection pointto the first heat exchanger where refrigerant exits the first heatexchanger during the defrost cycle; and a third connection point to thefirst heat exchanger where refrigerant is delivered from the compressorto the first heat exchanger during at least part of the defrost cycle; afirst refrigerant conduit connecting a discharge port on the compressorto the at least one first connection point of the first heat exchanger;a second refrigerant conduit connecting the second connection point ofthe first heat exchanger to the at least one expansion device; a thirdrefrigerant conduit connecting the first refrigerant conduit to thethird connection point of the first heat exchanger; a defrost valvelocated in the third refrigerant conduit between the first refrigerantconduit and the third connection point of the first heat exchanger,wherein, when the defrost valve is closed, refrigerant flow through thethird refrigerant conduit is blocked; and a control system that controlsthe defrost valve and opens the defrost valve during the defrost cycleallowing refrigerant to flow through the third refrigerant conduit tothe third connection point to defrost the first heat exchanger; wherein:the first connection point to the first heat exchanger is at the firstheader; the second connection point to the first heat exchanger is atthe second header; the third connection point to the first heatexchanger is at the second header; and refrigerant that, during at leastpart of the defrost cycle, passes through the third refrigerant conduit,through the defrost valve, and through the third connection point to thefirst heat exchanger, passes through the second header, heating thesecond header between the third connection point to the first heatexchanger and the second connection point to the first heat exchangerwithout passing through any cross tubes of the first heat exchanger. 2.The heat pump of claim 1 wherein the first heat exchanger comprises: atop and a bottom; and wherein the first header extends across the top ofthe first heat exchanger; the second header extends across the bottom ofthe first heat exchanger; the first header is horizontal; the secondheader is horizontal; and each of the multiple cross tubes directlyconnects to the first header, and directly connects to the secondheader.
 3. The heat pump of claim 1 wherein the first heat exchangerconsists essentially of: the first header; the second header; themultiple cross tubes; the multiple fins between the cross tubes, whereinthe fins are bonded to the cross tubes; the at least one firstconnection point to the first heat exchanger; the second connectionpoint to the first heat exchanger; and the third connection point to thefirst heat exchanger.
 4. The heat pump of claim 1 further comprising anextension tube located within the second header, wherein: the extensiontube within the second header is substantially parallel to the secondheader; and the third connection point to the first heat exchanger is atthe extension tube.
 5. The heat pump of claim 1 wherein: the first heatexchanger has only two headers, the first header and the second header.6. The heat pump of claim 1 wherein: the second header has a first endand a second end; each of the multiple cross tubes connects to thesecond header between the first end and the second end; the secondconnection point to the first heat exchanger is at the second end of thesecond header; and the third connection point to the first heatexchanger is at the first end of the second header.
 7. The heat pump ofclaim 6 wherein: the first header has a third end and a fourth end; eachof the multiple cross tubes connects to the first header between thethird end and the fourth end; and the at least one first connectionpoint to the first heat exchanger consists of a single first connectionpoint at the third end of the first header.
 8. The heat pump of claim 6wherein: the first header has a third end and a fourth end; each of themultiple cross tubes connects to the first header between the third endand the fourth end; the at least one first connection point to the firstheat exchanger comprises a primary first connection point to the heatexchanger at the third end of the first header and a secondary firstconnection point to the heat exchanger at the fourth end of the firstheader; and the first refrigerant conduit connects the discharge port onthe compressor to the primary first connection point and to thesecondary first connection point.
 9. The heat pump of claim 1 whereinthe control system comprises a digital controller comprising programminginstructions to open the defrost valve during the defrost cycle todefrost the first heat exchanger between the third connection point andthe second connection point, and wherein the digital controller furthercomprises programming instructions to keep the defrost valve closed whenthe heat pump is not in the defrost cycle.
 10. The heat pump of claim 9wherein the digital controller further comprises programminginstructions to keep the defrost valve closed during part of the defrostcycle to defrost the first heat exchanger between the at least one firstconnection point and the second connection point.
 11. The heat pump ofclaim 1 further comprising a first fan positioned and configured to moveair through the first heat exchanger, wherein the control systemcomprises a digital controller comprising programming instructions tooperate the first fan in a reversed direction during at least part ofthe defrost cycle to reduce natural convection through the first heatexchanger during the at least part of the defrost cycle.
 12. The heatpump of claim 1 further comprising a reversing valve located in thefirst refrigerant conduit between the discharge port on the compressorand the at least one first connection point of the first heat exchanger,wherein the third refrigerant conduit connects to the first refrigerantconduit between the reversing valve and the at least one firstconnection point of the first heat exchanger, the heat pump furthercomprising a second heat exchanger, a fourth refrigerant conduitconnecting the at least one expansion device to the second heatexchanger, a fifth refrigerant conduit connecting the second heatexchanger to the reversing valve, and a sixth refrigerant conduitconnecting the reversing valve to an inlet port on the compressor.
 13. Amethod of defrosting a first heat exchanger of a heat pump, the heatpump comprising the first heat exchanger, a compressor, at least oneexpansion device, and a second heat exchanger, the first heat exchangercomprising headers, multiple cross tubes, a first connection point tothe first heat exchanger, a second connection point to the first heatexchanger, and a third connection point to the first heat exchanger, themethod comprising, in any order except where a particular order isexplicitly indicated, at least the acts of: operating the heat pump in adefrost mode during a defrost cycle including delivering refrigerantfrom the compressor to the first connection point of the first heatexchanger; during the defrost cycle, passing the refrigerant through thefirst heat exchanger from the first connection point to the first heatexchanger, through the multiple cross tubes, to the second connectionpoint of the first heat exchanger; during the defrost cycle, passing therefrigerant from the second connection point of the first heatexchanger, through the at least one expansion device, and then to thesecond heat exchanger; during the defrost cycle, passing the refrigerantthrough the second heat exchanger, and then back to the compressor; andduring at least part of the defrost cycle, delivering at least part ofthe refrigerant from the compressor to the third connection point of thefirst heat exchanger; and passing the at least part of the refrigerantfrom the third connection point, through one of the headers, to thesecond connection point without passing the at least part of therefrigerant through any of the cross tubes of the first heat exchanger.14. The method of claim 13 wherein: the one of the headers of the firstheat exchanger comprises a first end and a second end; each of the crosstubes connect to the one of the headers between the first end and thesecond end; the second connection point of the first heat exchanger isat the second end of the one of the headers; the third connection pointof the first heat exchanger is at the first end of the one of theheaders; and the act of passing the refrigerant from the thirdconnection point, through the one of the headers, to the secondconnection point comprises passing the refrigerant from the first end,through the one of the headers, to the second end.
 15. The method ofclaim 13 wherein: each cross tube comprises multiple contiguous parallelrefrigerant passageways therethrough; the first heat exchanger furthercomprises multiple fins between the cross tubes that are bonded to thecross tubes; and the act of passing the refrigerant through the firstheat exchanger from the first connection point, through the multiplecross tubes, to the second connection point of the first heat exchangercomprises heating the multiple fins between the cross tubes.
 16. Themethod of claim 13 wherein the act of delivering refrigerant from thecompressor to the third connection point of the first heat exchangercomprises opening a solenoid valve in a bypass refrigerant lineextending from a supply refrigerant line connected to the firstconnection point, the bypass refrigerant line extending to the thirdconnection point.
 17. The method of claim 13 comprising, during a firstportion of the defrost cycle, not passing refrigerant through the thirdconnection point, and during a second portion of the defrost cycle,passing refrigerant through the third connection point.
 18. (canceled)19. (canceled)
 20. The method of claim 13 wherein: the headers consistof a first header and a second header; the first connection point to thefirst heat exchanger is at the first header; the second connection pointto the first heat exchanger is at the second header; the thirdconnection point to the first heat exchanger is at the second header;and the act of passing the at least part of the refrigerant from thethird connection point, through one of the headers, to the secondconnection point comprises passing the at least part of the refrigerantthrough the second header without passing the at least part of therefrigerant through any of the cross tubes of the first heat exchanger.21. The method of claim 20 wherein: the first heat exchanger is anoutdoor air heat exchanger; the second heat exchanger is an indoor airheat exchanger; the first heat exchanger comprises a top and a bottom;the first header extends across the top of the first heat exchanger; thesecond header extends across the bottom of the first heat exchanger;each cross tube of the first heat exchanger comprises multiplecontiguous parallel refrigerant passageways therethrough; each of themultiple cross tubes directly connects to the first header; each of themultiple cross tubes directly connects to the second header; the firstheat exchanger further comprises multiple fins between the cross tubesthat are bonded to the cross tubes; and the act of passing therefrigerant through the first heat exchanger from the first connectionpoint, through the multiple cross tubes, to the second connection pointof the first heat exchanger comprises heating the multiple fins betweenthe cross tubes; the act of delivering refrigerant from the compressorto the third connection point of the first heat exchanger comprisesopening a solenoid valve in a bypass refrigerant line extending from asupply refrigerant line connected to the first connection point, thebypass refrigerant line extending to the third connection point.
 22. Aheat pump comprising: a compressor; at least one expansion device; afirst heat exchanger comprising a top and a bottom and consistingessentially of: a first header extending across the top of the firstheat exchanger; a second header extending across the bottom of the firstheat exchanger; and multiple cross tubes extending from the first headerto the second header, wherein: each of the multiple cross tubes isdirectly connected to the first header; each of the multiple cross tubesis directly connected to the second header; and the multiple cross tubeseach include multiple contiguous parallel refrigerant passagewaystherethrough; multiple fins between the cross tubes wherein the fins arebonded to the cross tubes; at least one first connection point whererefrigerant is delivered to the first heat exchanger from the compressorduring the defrost cycle; a second connection point where refrigerantexits the first heat exchanger during the defrost cycle; and a thirdconnection point where refrigerant is delivered from the compressor tothe first heat exchanger during at least part of the defrost cycle; afirst fan positioned and configured to move air through the first heatexchanger; a second heat exchanger; a first refrigerant conduitconnecting a discharge port on the compressor to the at least one firstconnection point of the first heat exchanger, wherein the firstrefrigerant conduit does not include any part of the first heatexchanger; a reversing valve located in the first refrigerant conduitbetween the discharge port on the compressor and the at least one firstconnection point of the first heat exchanger; a second refrigerantconduit connecting the second connection point of the first heatexchanger to the at least one expansion device, wherein the secondrefrigerant conduit does not include any part of the first heatexchanger; a third refrigerant conduit connecting the first refrigerantconduit to the third connection point of the first heat exchanger; adefrost valve located in the third refrigerant conduit between the firstrefrigerant conduit and the third connection point of the first heatexchanger, wherein, when the defrost valve is closed, refrigerant flowthrough the third refrigerant conduit is blocked; and a fourthrefrigerant conduit connecting the at least one expansion device to thesecond heat exchanger; a fifth refrigerant conduit connecting the secondheat exchanger to the reversing valve; a sixth refrigerant conduitconnecting the reversing valve to an inlet port on the compressor; acontrol system that controls the defrost valve and opens the defrostvalve during the defrost cycle allowing refrigerant to flow through thethird refrigerant conduit to the third connection point; wherein: thecontrol system comprises a digital controller comprising programminginstructions to open the defrost valve during the defrost cycle todefrost the first heat exchanger between the third connection point andthe second connection point; the digital controller further comprisesprogramming instructions to keep the defrost valve closed when the heatpump is not in the defrost cycle; the third refrigerant conduit connectsto the first refrigerant conduit between the reversing valve and the atleast one first connection point of the first heat exchanger; the firstconnection point is at the first header; the second connection point isat the second header; the third connection point is at the secondheader; and refrigerant that, during at least part of the defrost cycle,passes through the third refrigerant conduit, through the defrost valve,and through the third connection point, passes through the secondheader, heating the second header between the third connection point andthe second connection point without passing through any cross tubes ofthe first heat exchanger.
 23. The heat pump of claim 22 wherein: thesecond header has a first end and a second end; each of the multiplecross tubes connects to the second header between the first end and thesecond end; the second connection point to the first heat exchanger isat the second end of the second header; and the third connection pointto the first heat exchanger is at the first end of the second header.